Gaming Ballistic and Don't Forget Your Boots wrote about some of the oddness that comes from turn-based games and the intersection of some situations and options.
Naturally, after playing for so long, my group and I have had a few of these weird moments, courtesy of odd circumstances and a turn-based resolution system.
A couple come to mind.
Vryce and the Orc Spellcaster
In a big orc fight, the orcs tried to plug the doorway. Vryce burst through and ran through a small gap in the ranks to attack a back-ranks spellcaster. On the spellcaster's turn, he ran away. Then orcs poured in to prevent Vryce from following. Vryce's player was frustrated - how could someone run 5 yards away from him in the blink of an eye, after being within sword's reach a split second before?
But Vryce even getting that strike was possibly an artifact of turn-based play. In a real-time simultaneous-move world, Vryce was only marginally faster than the orcs. The spellcaster was already intending to fall back until he had more orcs between him and the well-known lethal threat that is Vryce. More orcs were charging in to seal off the gap. He might not have been able to bust through the momentary gap in the ranks, because that moment would appear and pass in a split second. The spellcaster may have moved out of reach right away. Instead, you get this moment of "I'm just fast enough to bust through and get a wild swing at that guy as he runs, before his buddies cut me off and he moves out of reach." That's how I interpret that, and that's pretty reasonable. It makes the tactical imperative of physically filling space, not just threatening to fill it with a swing or a step, much more clear.
On the day, I remember saying that if Vryce basically had a Zone of Control (aka ZOC) or a zone of opportunity attacks, he would have been able to prevent the spellcaster from moving freely or to get a free cut at that spellcaster. But then again, so would all of those orcs he deftly ran by because of a few open hexes in exactly the right place for him to move. He didn't need Evade, but "hexes adjacent to an enemy cost extra to enter or leave" or "opportunity attacks" would have kicked in. I suppose we could make rules like that, but they'd substantially change the rules set, and probably replace the current oddness (run up and hit, then the guy runs out of reach) with more oddness (how do you enforce a ZOC in your front arc with a one-action, one-second timescale? How exactly are you doing this without physically restraining the target? If you have time and movement to execute more attacks, why aren't you using them in the first place?)
We've had the classic "runaround" attack issue discussed elsewhere very often. Sometimes, it only looks like a runaround. One classic was with a sword-and-shield using soldier-wizard-engineer in my previous GURPS game. The group was fighting gnolls in a mountainside keep (in UK3 The Gauntlet, actually). One PC knowingly and willingly turned his back on the gnolls to get a shot at a foe close and behind the line of PCs. So a greataxe-armed gnoll used All-Out Attack and smacked him. The player was miffed he didn't get to defend. "I knew he was there!" and "I should defend at -2!" kind of stuff (not a quote, it's been 16 years since that session.) I said, yes, you knew he was there. You knew there were dangerous people in front of you and turned your back on them, on your turn, to do something. This wasn't a case of a turn-based system artifact causing a faster foe to get a back shot. This was a terrible tactical decision to turn his back on a foe he thought was safely out of reach, forgetting that he was just within reach of a Move 6 guy's attack if he had a 2-reach axe ready at full extension . . . and he did. Bam, roll up a new PC.
Could he have looked over his shoulder? I suppose, but is the standard of combat "I'm checking my 6 over each shoulder every turn with no cost for taking my eyes off of what's in front of me" and complete awareness? Not to me. I figure the "runaround" rule, and the way we usually rule it (only if a foe starts in your back hex, and generally only if it's reasonable that you couldn't or wouldn't or just plain didn't track him visually) is fine. It's clear most of the time. This is one of those clear cases - he turned his back, assuming he could do so without consequence, and was wrong.
We've had related cases thanks to people making odd choices of where to step, wearing blinders (I mean, a greathelm), or having one eye. Most of the time, people accept that, yes, I've made my own decisions (stepping poorly, facing away from a foe, wearing a vision-restricting helmet because I want more DR) and I'll take the consequences. Occasionally, though, we get the "I'm checking over both shoulders while keeping an eye on this guy in front of me and Feinting the dude next to him" approach, and grumpiness can ensue when I rule in favor of a free back shot.
How do I rule on them?
Basically, we play the rules as written. But I'm not hesitant to make one-time rulings for situations where they seem to warrant it.
I also tend to apply my own vision of the situation and the reasonableness of results, along with the fairness (witness the "If bad guys can't interrupt Great Haste with Wait, you can't either" ruling.)
If you make moves depending on what a literal reading of the rules say you can and can't do, and what must result from that, you are taking a risk. If you make moves based on what a reasonable person would think would happen in those circumstances, and assume that generally I'm a reasonable GM, you will be okay. It's the latter standard that I use, not the former, to determine what's okay in a situation. You're arguing before a judge who considers precedent but who isn't bound by it. Sometimes, the rules say you get a defense at -2, but circumstances say you don't. Sometimes, the rules say you don't, but circumstances say you do.
And I'm okay with that. That's part of the reason for a GM, so we can have rules and games that work that way.